Background: Cholesterol and bile acids have been implicated in playing important roles in several major diseases of "Western Society" including: arteriosclerosis, cholesterol gallstone disease, cholestatic liver diseases, and colon cancer. The overall goal of this renewal application is aimed at a more detailed understanding of how the body regulates bile acid and cholesterol homeostasis, liver/intestinal physiology and determining if secondary bile acids are involved in the risk of cholesterol gallstone disease. The overall goal will be accomplished through the following specific aims: a) determine which cell signaling pathways are activated by bile acids in primary hepatocytes and which are important in regulating genes involved in cholesterol metabolism and phospholipid transport; b) determine if JNK-1 and JNK2 null mice are defective in cholesterol homeostatic mechanisms and if bile acid activated cell signaling pathways "cross-talk" with bile acid activated nuclear receptors e.g., FXR (Dent, Hylemon); c) characterize in detail the FTF/HNF-4 site in the sterol 12alpha-hydroxylase (CYP8bl) promoter and elucidate the molecular mechanism by which FTF/SHP specifically regulates CYP8b1 transcription; d) characterize the molecular mechanism involved in the SREBP-2 mediated suppression of the CYP8B1 promoter; e) characterize the significance and physiological role SREB-2-mediated suppression of CYP8b1 (Gil); f) determine the role of steroidogenic acute regulatory (StAR) protein and other intracellular cholesterol transport proteins (SCP-2, MLN64) play in the regulation of bile acid synthesis in the liver; g) determine if StAR is expressed in the liver (Pandak,Gil); h) determine the 3 dimensional (3D) structure and catalytic mechanism of bile acid 7alpha and 7beta-dehydratase from Clostridium scindens; i) express, purify, and characterize a novel 3-oxo-delta4steroid oxidoreductase from C. scindens; j) clone, sequence, and analyze the bai operon from Clostridium hylemonae TN271; k) isolate, characterize, and identify cholic acid 7alpha-dehydroxylating bacteria from cholesterol gallstone patients with high (>30%) deoxycholic acid and controls; and I) determine if gallstone patients are colonized by unique species of 7alpha-dehydroxylating bacteria. (Hylemon, Heuman).